A fundamental problem in neuroscience is to understand events occurring within Individual neurons and within neural networks that contribute to forms of plasticity underlying learning and memory. This proposal outlines both empirical and modeling studies that will examine the molecular, biochemical and biophysical properties of Identified neurons and the connectivity of neural circuits that have demonstrated capacities for nonassociative and associative plasticity. Specifically, the neural circuit that mediates the tail withdrawal reflex will be analyzed. Many of the sensory neurons, Interneurons, motor neurons and modulatory interneurons that control this behavior have been Identified and are accessible to study. Thus, molecular, biochemical and cellular neurophysiological techniques will be applied to analyze the particular processes that might explain associative and nonassociative learning. Formalisms of the cellular and network processes that underlie these forms of plasticity will be developed and incorporated into quantitative, real-time models of neuron-like elements and neural networks. The ability of these models to fit the experimental data and to predict simple and complex features of learning will be examined. The proposed research will provide for a fairly complete analysis of the mechanisms underlying the Induction, expression and maintenance of simple forms of nonassociative and associative learning as well as help address fundamental questions regarding the mechanistic relationship between short- and long-term memories.